adding new variable without extending class

Question

I have button and I'd like to do something like this:

Button b=new Button(){public int a;};
b.a=5;

How to do this in java?

Answer 1

There is no way to add variables to a class without extending it. However, the extending class does not need to be named: you could use an anonymous class instead, in a way similar to the snippet from your post.

The trick, however, is to access the variable after you have added it: you could do it by mutating objects referenced from the class, like this:

// Prepare a mutable object for use in your class
final AtomicInteger a = new AtomicInteger(123);
Button b = new Button(){
    public void someMethod() {
        ...
        int n = a.intValue();
        ...
    }
};

When you set things up this way, changes to a's state in your method would become accessible to someMethod() of the anonymous subclass of the Button class.

Answer 2

It seems like you want a custom Button object.

Simply create a new Class with extends Button and declare in that class the a variable and any other methods that you want to add.

Something like

public class SomeClass extends Button
{
   public int a;
}

public class SomeOtherClass
{
    public static void main(String args[])
    {
        SomeClass someClass = new SomeClass();
        someClass.a = 5;
    }
}

If really you don't want to extend the class, then it is impossible. Sorry.

Also, your example is actually creating an anonymous class which extends Button.